I filed an RTI to know the official information about the OLDEST TRAINS from each zone.
The official replies I got are compiled in this list.
1. CR: Yet to receive
2. ECR (10.2002): 5529/30 Banmankhi - Samastipur Express (03.11.2002) now 55533/4 Banmankhi - Samastipur Passenger after gauge conversion.
3....
more...

ECoR: 8415/6 Bhubaneswar - Yesvantpur Express (01.04.2003) now running as 12845/6 Bhubaneswar - Bengaluru Cantt. SF Express
4. ER: First train between Howrah and Hooghly on 15.08.1854 and 1/2 Calcutta - Delhi in 1866 now running as 12311/2 Kalka Mail
5. NCR (2002): 2451/2 Shram Shakti SF Express (01.09.2002) now 12451/2 Shram Shakti SF Express
6. NER: Yet to receive
7. NWR (10.2002): 1JS/2JS 3/4JS Jodhpur Passenger (14.12.2002)
8. NFR: Information not available in the office.
9. NR: Information not available in the office.
10. SCR (02.10.1966): 1/2 Secunderabad - Vijayawada Golconda Express on (01-03-1969) now running as 17201/2 Golconda Express
11. SECR (01.04.2003): 405/6B Bilaspur - Bishrampur Express (01.07.2003) now running as 18241/2 Durg - Ambikapur Express
12. SER: 95/6 Howrah - Bombay Deluxe Express; 99/100 Howrah - Madras Deluxe Express; 457/8 Rourkela - Titlagarh Passenger; 1PA/2PA Adra - Purulia Passenger now running as 63593/4 Purulia - Asansol MEMU (01.04.1967); 1DR/2DR 3DR/4DR Durg - Raipur Passenger (01.10.1967) now under SECR.
13. SWR (2003-04): 7311/2 Madras Vasco Weekly Express (15.12.2003) now running as 17311/2 Chennai Vasco Weekly Express
14. SR: Information not available in the office.
15. WCR: Yet to receive
16. WR: Information not available in the office.
17. KR: Yet to receive